Earth formation drilling utilises a long string of drilling pipes and tools coupled together. All elements of the drilling string are rotated together in order to rotate a cutting bit at the end of the drilling sting. The cutting bit creates a hole in a formation through which the rest of the drilling string moves in a drilling direction. An under-reamer, coupled between two other elements of the drilling string, is used to widen the walls of the hole created by the drill bit. The under-reamer, also known as a reamer, normally has an overall diameter in its retracted position which is the same as or less than the diameter of the hole being drilled. When in its deployed position, cutting elements are moved away from the body of the under-reamer to define a diameter which is larger than the diameter of the hole being drilled. As the under-reamer moves downhole rotating with the drilling string, it widens the hole in the formation behind the drill bit. In addition, an under-reamer may be used to open a collapsed formation on its way back up to the surface.
WO-A-2005/124094 describes one such under-reamer or reamer tool. The reamer tool comprises a tubular body having an axial cavity and housings arranged around its periphery to define external openings. In each of these openings, a cutter element is housed which comprises two cutter arms that can be moved between a retracted position where each cutter element is fully retained within its associated housing, and an expanded position where each cutting element extends outside its opening so that more material can be cut away the walls of the hole in a formation thereby enlarging its diameter. A drive mechanism is provided within the tubular body to move the cutter elements between their retracted and expanded positions.
In the reamer tool described in WO-A-2005/124094, one cutter arm is pivotally connected to the tubular body at one end and to the other cutter arm at the other end, the other cutter arm being connected to the drive mechanism so that both cutter arms can be retracted and expanded. The arrangement formed by the two cutter arms when deployed is a ‘V’-shape where the vertex of the V is outside the opening.
Typically, such reamer tools are operated by the pressure of fluid passing through the drill string, and in particular, through the tool section itself. The pressure of fluid is controlled by the operation of a pump associated with the drill string. In US-A-2010/0006339, the pressure of fluid passing through the tool is used to operate the reamer so that it is expanded or retracted in accordance therewith. Here, the reamer assembly comprises cutter elements and stabiliser pads mounted for sliding movement on grooves. In the retracted position, the reamer assembly is housed within a recess, the reamer assembly being moved to the expanded position by movement along the grooves so that it is outside the recess. Fluid pressure is sensed to activate the expansion and retraction of the reamer.
US-A-2010/0096191 discloses an under-reaming and stabilisation tool in which a blade element is moved from a retracted position to an expanded position by wedge elements coupled to a drive tube, the wedge elements interact with an inclined face of the blade element to effect the raising (expansion) and lowering (retraction) of the blade element relative to a guide channel. As the drive tube moves along the length of the tool body, the wedge elements are drawn along therewith and they slide under the inclined face of the blade element causing radial movement of the blade element to raise out (expand) out of its guide channel. Movement of the drive tube in the opposite direction along the length of the tool body withdraws the wedge elements from under the inclined face of the blade element allowing radial movement of the blade element to lower (retract) into its guide channel. The expansion of the blade element is limited by the actuation mechanism, that is, the drive tube and wedge elements coupled thereto.